The present invention is directed to the utilization of carbo- and heterocyclic spiro compounds as donor materials in the fabrication of organic photovoltaic (OPV) devices.
OPV devices have attracted enormous interest over the last two decades because of their great technological potential as a renewable, alternative source for electrical energy. Significant advances on the development of smart materials and innovative device engineering have boosted the power conversion efficiencies (PCE) up to 9.2% for a single cell and up to 10.6% for a tandem device. Up until now, most of the OPV devices with record-high PCEs are based on the use of newly synthesized polymers or oligomers as donor materials in combination with fullerene acceptors (Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photonics 2012, 6, 591; J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. Chen, J. Gao, Y. Yang, Nat. Commun. 2013, 4, 1446). However, the major challenges for fabricating highly efficient polymeric OPV devices include the difficulty to produce high quality thin films as well as to achieve high batch-to-batch reproducibility. In addition, it is very difficult to control the morphology and phase separation of the blended active layer, which plays a vital role in determining the overall device performance. On the other hand, small molecules are monodispersed in nature with well-defined chemical structure and are synthetically well reproducible with high purity. In comparison to polymer-based thin films, it is also much easier to produce high quality and uniform thin films based on small molecules by vacuum deposition.
While small molecule-based OPV devices can in principle have higher performance and longer stability, the development of small molecule-based OPV devices has received less attention owing to their relatively lower PCEs compared to those of their polymer counterparts. The first small molecule-based bilayer OPV device was demonstrated by Kearns and Calvin in 1958, in which a reasonably high photovoltage of 200 mV with a power output of 3×10−12 W was obtained for a bilayer magnesium phthalocyanine and oxidized tetramethyl-p-phenylenediamine device (D. Kearns, M. Calvin, J. Chem. Phys. 1958, 29, 950). In 1986, Tang reported a highly efficient bilayer OPV device based on copper phthalocyanine (CuPc) as donor and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), an analogue of perylene bisimide, as acceptor to form a p-n junction. A PCE of approximately 1% with a high fill factor (FF) of 0.65 was recorded under simulated Air Mass (AM) 2 illumination (C. W. Tang, Appl. Phys. Lett. 1986, 48, 183). This successful demonstration was believed to be due to the careful choice of photoactive materials. In particular, a large energy offset between the CuPc donor and the PTCBI acceptor at the organic heterojunction provided a sufficiently large driving force for efficient exciton dissociation, yielding a dramatic PCE enhancement as compared to single layer devices.
This breakthrough can be traced back to the discovery of fullerene which exhibits excellent electron-transporting property and can be applied as an acceptor material in OPV devices (N. S. Sariciftci, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger, G. Stucky, F. Wudl, Appl. Phys. Lett., 1993, 62, 585). Since then, there have been numerous reports of high efficiency OPV devices utilizing fullerene derivatives as acceptor materials. Fullerene derivatives are the most promising candidates as acceptor materials in OPV devices. However, there still remains a need for the development of good hole-transporting materials as donors in combination with fullerene to result in an efficient OPV device.
Spirobifluorene and its derivatives are among the most well-known classes of hole-transporting materials owing to their high hole-mobilities of up to the order of 10−3 cm2 V−1 s−1. The spiro-linkage between two fluorene units with a sp3-hybridized carbon atom can lead to the perpendicular arrangement of two halves, giving structural rigidity. In addition, such three-dimensional (3D) character of spirobifluorene can increase its solubility and glass transition temperature (Tg) (T. P. I. Saragi, T. Spehr, A. Siebert, T. F. Lieker, J. Salbeck, Chem. Rev. 2007, 107, 1011). The conjugation along the horizontal orientation of the molecules can also increase the intermolecular charge transport property in vacuum-deposited thin films (C. Wu, W. Liu, W. Hung, T. Liu, Y. Lin, H. Lin, K. Wong, Y. Chien, R. Chen, T. Hung, T. Chao, Y. Chen, Appl. Phys. Lett. 2005, 87, 052103). Inspired by this structural feature, spirobifluorene and its derivatives have been widely employed as hole-transporting layers in organic light-emitting devices (OLEDs).
Compared with the large number of publications exploring the use of spirobifluorenes as hole-transporting materials in OLEDs, there is less attention on their use in fabricating OPV devices. Indeed, spirobifluorene has a very low absorption coefficient within the visible spectrum and could be harmful to the photovoltaic process. Recently, the possibility of using spirobifluorene as a building block with anchoring groups to construct sensitizers for OPV devices has been explored (D. Heredia, J. Natera, M. gervaldo, L. Otero, F. Fungo, C.-Y. Lin, K.-T. Wong, Org. Lett. 2010, 12, 12; H.-C. Ting, C.-H. Tsai, J.-H. Chen, L.-Y. Lin, S.-H. Chou, K.-T. Wong, T.-W. Huang, C.-C. Wu, Org. Lett. 2012, 14, 6338; G. Pozzi, S. Orlandi, M. Cavazzini, D. Minudri, L. Macor, L. Otero, F. Fungo, Org. Lett. 2013, 15, 4642; Q. Yan, Y. Zhou, Y.-Q. Zheng, J. Pei, D. Zhao, Chem. Sci. 2013, DOI: 10.1039/c3sc5184h). The sterically bulky spiro core can effectively reduce the intermolecular interactions and suppress aggregation-induced self-quenching between the molecules. Meanwhile, an efficient alternating copolymer has been synthesized by incorporating a spirobifluorene core with a benzothiadiazole unit (M. Wang, C. Li, A. Lv, Z. Wang, Z. Bo, Macromolecules 2012, 45, 3017). Such copolymer exhibited high hole mobility of up to 7×10−3 cm2 V−1 s−1, and a high PCE of up to 4.6% has been achieved for devices based on the copolymer/fullerene blend. While spirobifluorene is a useful building block for designing efficient donor materials, there is scarcely any literature report on utilizing the heterocyclic spiro compounds in the fabrication of OPV devices.